1. Field of the Invention
This invention relates to noble metal compositions for producing solderable conductor patterns which are adherent to substrates.
2. Description of the Prior Art
Conductor compositions which are applied to and fired on ceramic substrates, or on ceramic substrates provided with a thick film dielectric layer, usually comprise finely divided noble metal particles and inorganic binder particles. Such compositions are commonly applied to substrates as a "thick film", a dispersion of the inorganic materials of the compositions in an inert liquid medium or vehicle. The metallic component of the composition provides an electrical conductor having low electrical resistance while the inorganic binders, such as glass, bonds the metal particles to the substrate.
The most relevant prior art known to Applicants is that found in U.S. Pat. No. 3,970,590 which issued on July 20, 1976. This patent discloses gold conductor compositions which can by conventional thick film techniques produce conductor patterns and pads to which electronic components can be bonded using conventional thermal compression bonding techniques. The particular gold conductor compositions disclosed in the Patent 3,970,590 have excellent adherence to ceramic substrates and such substrates coated with thick film dielectric layers composed, for example, of mixtures of alumina and glass. However, the strength of the solder bond, or joint between a pad made of the composition, a lead pad, and a copper lead precoated with solder varies from being unacceptably weak to being acceptably strong.
The major problem with the prior art gold conductor composition is that the strength of solder joints between lead pads which are made from some compositions within the range disclosed in U.S. Pat. No. 3,970,590 and copper electronic leads soldered to the pads with a 50% lead, 50% indium solder have widely varying strengths with some being unacceptable weak both initially and after being aged at elevated temperatures.
It is therefore an object of this invention to provide a solderable gold conductor composition that adheres to fired dielectric surfaces of ceramic substrates and to the surface of ceramic substrates without dielectric surfaces.
It is another object of this invention to provide an improved solderable gold conductor composition in which the conductor composition does not need to be physically or chemically cleaned prior to soldering beyond the action of noncorrosive fluxes compatible with the reliability requirements of electronic systems such as digital computers.
It is still another object of this invention to produce an improved solderable gold conductor composition which optimizes the strength of solder bonds between leads of electronic components and the portions, or lead pads of the composition to which such leads are soldered while retaining the characteristic that the pattern does not need to be physically or chemically cleaned prior to soldering.
It is yet another object of this invention to provide an improved method of soldering leads to lead pads on a substrate.